Scientists who have long been puzzled about the purpose of sleep say it may be the brain's way of flushing toxins out.
Cells shrink as we sleep, to allow waste to be removed more effectively
Humans need sleep so that the brain can flush toxic waste out of its cells, new research has shown.
The findings, published in the journal Science, show that the brain's unique method of cleansing itself, known as the glymphatic system, is highly active during sleep.
As we slumber, it clears away toxins that would otherwise build up and possibly trigger neurological disorders such as Alzheimer's disease.
The purpose of sleep has puzzled both philosophers and scientists since ancient Greek times.
However, the latest research suggests that during sleep, brain cells reduce in size to allow waste to be removed more effectively.
The findings indicate that waste disposal may underlie the restorative properties of sleep.
There could also be far reaching implications for understanding and treating disease.
"This study shows that the brain has different functional states when asleep and when awake," said Dr Maiken Nedergaard, a researcher at the University of Rochester.
"In fact, the restorative nature of sleep appears to be the result of the active clearance of the by-products of neural activity that accumulate during wakefulness.
"The brain only has limited energy at its disposal and it appears that it must choose between two different functional states - awake and aware or asleep and cleaning up.
"You can think of it like having a house party. You can either entertain the guests or clean up the house, but you can't really do both at the same time."
Why do we sleep? To clean our brains, say US scientists
Cerebral spinal fluid found to pump around the brain of sleeping mice, flushing out waste products like a biological dishwasher
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Scientists say they may have found the reason why we need to sleep. Photograph: Corbis
Scientists in the US claim to have a new explanation for why we sleep: in the hours spent slumbering, a rubbish disposal service swings into action that cleans up waste in the brain.
Through a series of experiments on mice, the researchers showed that during sleep, cerebral spinal fluid is pumped around the brain, and flushes out waste products like a biological dishwasher.
The process helps to remove the molecular detritus that brain cells churn out as part of their natural activity, along with toxic proteins that can lead to dementia when they build up in the brain, the researchers say.
Maiken Nedergaard, who led the study at the University of Rochester, said the discovery might explain why sleep is crucial for all living organisms. "I think we have discovered why we sleep," Nedergaard said. "We sleep to clean our brains."
Writing in the journal Science, Nedergaard describes how brain cells in mice shrank when they slept, making the space between them on average 60% greater. This made the cerebral spinal fluid in the animals' brains flow ten times faster than when the mice were awake.
The scientists then checked how well mice cleared toxins from their brains by injecting traces of proteins that are implicated in Alzheimer's disease. These amyloid beta proteins were removed faster from the brains of sleeping mice, they found.
Nedergaard believes the clean-up process is more active during sleep because it takes too much energy to pump fluid around the brain when awake. "You can think of it like having a house party. You can either entertain the guests or clean up the house, but you can't really do both at the same time," she said in a statement.
According to the scientist, the cerebral spinal fluid flushes the brain's waste products into what she calls the "glymphatic system" which carries it down through the body and ultimately to the liver where it is broken down.
Other researchers were sceptical of the study, and said it was too early to know if the process goes to work in humans, and how to gauge the importance of the mechanism. "It's very attractive, but I don't think it's the main function of sleep," said Raphaelle Winsky-Sommerer, a specialist on sleep and circadian rhythms at Surrey University. "Sleep is related to everything: your metabolism, your physiology, your digestion, everything." She said she would like to see other experiments that show a build up of waste in the brains of sleep-deprived people, and a reduction of that waste when they catch up on sleep.
Vladyslav Vyazovskiy, another sleep expert at Surrey University, was also sceptical. "I'm not fully convinced. Some of the effects are so striking they are hard to believe. I would like to see this work replicated independently before it can be taken seriously," he said.
Jim Horne, professor emeritus and director of the sleep research centre at Loughborough University, cautioned that what happened in the fairly simple mouse brain might be very different to what happened in the more complex human brain. "Sleep in humans has evolved far more sophisticated functions for our cortex than that for the mouse, even though the present findings may well be true for us," he said.
But Nedergaard believes she will find the same waste disposal system at work in humans. The work, she claims, could pave the way for medicines that slow the onset of dementias caused by the build-up of waste in the brain, and even help those who go without enough sleep. "It may be that we can reduce the need at least, because it's so annoying to waste so much time sleeping," she said.
